E129 Boiler Fault Code: What It Means on Every Brand

The E129 fault code appears on 2 boiler brands — Baxi and Potterton — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What E129 means	Severity	DIY?
BBaxi	The E129 fault code indicates a safety lockout caused by the boiler repeatedly failing to hold a flame during its ignition calibration sequence, even after applying the maximum gas… Full guide →	Lockout — engineer likely	No
PPotterton	The E129 fault code on Potterton boilers signals a flame detection problem — specifically, that the boiler is either losing its flame too frequently to be considered safe, or (on some… Full guide →	Lockout — engineer likely	No

E129 brand by brand

Baxi E129

The E129 fault code indicates a safety lockout caused by the boiler repeatedly failing to hold a flame during its ignition calibration sequence, even after applying the maximum gas correction it is capable of making. Unlike the related E128 code — which flags flame loss during normal running — E129 is specific to the calibration phase that the boiler runs through as part of its start-up routine. Because the boiler cannot safely establish and confirm a stable flame within this sequence, it shuts itself down as a precaution. This fault points toward a problem in the ignition or gas delivery system and will need a Gas Safe registered engineer to properly diagnose and resolve.

Causes, fixes & costs for Baxi E129 → · All Baxi codes

Potterton E129

The E129 fault code on Potterton boilers signals a flame detection problem — specifically, that the boiler is either losing its flame too frequently to be considered safe, or (on some models such as the Titanium and Gold ranges) that the flame dropped out during the boiler's automatic combustion calibration process. Either way, the boiler cannot reliably sustain a flame, so it locks out as a precaution. Modern Potterton boilers are designed to shut down in exactly this situation, because continuing to run with an unstable flame could create a dangerous condition. You may be able to restart the boiler with a reset, but if the fault keeps returning you'll need a Gas Safe registered engineer to investigate the root cause.

Causes, fixes & costs for Potterton E129 → · All Potterton codes

Frequently asked questions

Is the E129 fault code the same on every boiler?

Broadly, yes — on Baxi and Potterton the E129 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a E129 fault myself?

Not usually. On every brand listed, E129 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a E129 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.